This invention relates to a calibration system for a magnetic heading sensor, or compass, in a vehicle, and, more particularly to a continuous and fully automatic calibration system for the compass in the vehicle. The invention can be applied to a magneto-inductive sensor, magneto-resistive sensor, flux gate sensor and other known sensing technology.
Many vehicles today are equipped with magnetic compasses to determine the direction in which the vehicle is heading and convey such information to the passengers and/or driver of the particular vehicle. Generally, these magnetic compasses include a magneto-responsive sensor, such as a magnetic rotor sensor, a flux gate sensor, a magneto-resistive sensor, a magneto-inductive sensor, a magneto-capacitive sensor, or the like. The magneto-responsive sensor detects the magnetic field present in the vicinity of the vehicle and processes this signal in order to determine a directional heading of the vehicle relative to the Earth magnetic field. However, these magnetic compasses must be calibrated within the vehicle to account for any deviating magnetic field from the vehicle or other structures surrounding the vehicle, in order to determine the true heading of the vehicle relative to the Earth magnetic field. Additionally, as the deviating field may change over time, due to either a change in the magnetic signature of the vehicle or in the magnetic field surrounding the vehicle, an original calibration may later become less accurate.
To date, there have been compasses proposed which include automatic magnetic compass calibration within a vehicle after an initial manual adjustment or preset has been made to the compass system. Such calibration processes are performed as the vehicle is driven in order to account for changes in the deviating magnetic fields, thereby maintaining an accurate directional readout to the driver of the vehicle. However, many of these calibration systems require the vehicle to be oriented in specific directions relative to Earth magnetic field, such as orienting the vehicle in 180 degree opposite directions or driving the vehicle in a complete 360 degree circle while data is sampled by the system, in order to calibrate the system. Many other systems alternatively require complicated mathematical functions to determine the true Earth magnetic field based on several data points collected as the vehicle is driven in several directions. While these systems may provide an accurate calibration in many areas of the Earth, they are often based upon an assumption that a trace or plot of the Earth magnetic field is substantially circular in shape, as plotted on a Cartesian coordinate system relative to the vehicle. In reality, however, irregularities may occur in the mounting of the compass and/or sensors in the vehicle such that the sensors may be tilted relative to the magnetic field of the Earth. Furthermore, the variations or declinations present in the Earth magnetic field at any given location are generally not perfectly symmetrical, as the declination varies irregularly over the Earth surface and further varies over a period of time. These irregularities and variations may result in a substantially non-circular or oval-shaped trace of the magnetic field rather than the circular field that many of the proposed calibration systems are based on.
An additional concern with the systems proposed to date is that the initial deviating magnetic field or magnetic signature of the vehicle must be offset so that the magneto-responsive sensor's output will be within an operable range of the electronic processing system. As a vehicle is manufactured, or shortly thereafter, the deviating field of the vehicle may be substantially offset or nullified by an initial preset of the vehicle's magnetic signature, which brings an origin of the Earth magnetic field to within a predetermined range of a center or origin of the compass coordinate system. Once the vehicle has its magnetic signature preset, the algorithmic or digital calibration systems may be implemented to refine the vehicle compass to within a desired range of accuracy.
Furthermore, the magnetic signature of a vehicle may change significantly over time, such as when a new sunroof motor is installed in the vehicle, a magnetic antenna is mounted to the vehicle or the like. If the magnetic signature changes too much, the Earth magnetic field, as sensed by the sensors, may be shifted out of the operable range of the analog-to-digital converter of the calibration system. In order to re-set the deviating magnetic field of the vehicle such that the sensed Earth field is back within the window of the calibration systems, the compass system may again need to be manually adjusted by a mechanic or technician.
Therefore, there is a need in the art for a fully automatic and continuous calibration system for calibrating a magnetic compass located on a vehicle. The calibration system must be able to account for the deviating magnetic field of the vehicle without requiring an initial preset or demagnetization of the vehicle as the vehicle is assembled. Furthermore, the calibration system must continuously account for minor and major changes in the deviating magnetic field by digitally or physically adjusting for such changes. These adjustments must also account for both circular and non-circular Earth magnetic fields. Furthermore, the calibration system must account for major changes in the deviating magnetic field in order to avoid requiring manual adjustments throughout the life of the vehicle.